CENSUS – Chlorinated Biphenyls

From 1929 to 1978, polychlorinated biphenyls (PCBs) manufactured under the commercial names like Aroclor, Fenclor, Kanechlor and Phenclor were widely used as transformer and hydraulic fluids (Field and Sierra-Alvarez 2008).  Commercial PCB preparations are mixtures composed of 60 to 90 congeners with different degrees and positions of chlorine substituents.   For example, Aroclor 1242, 1248, 1254, and 1260 are PCB mixtures with average chlorine weight percentages of 42, 48, 54, and 60%.  Both the degree of chlorination and the positions of the chlorines (orthometa, or para) impact PCB biodegradation.  In general terms, highly chlorinated biphenyls are subject to reductive dechlorination while less heavily chlorinated congeners can be co-metabolized aerobically.  Thus, while considered persistent in part due to their hydrophobicity, PCBs can potentially be mineralized through a sequence of anaerobic-aerobic biodegradation.

Reductive Dechlorination of PCBs

Molecular evidence had conclusively implicated strains of Dehalococcoides in the reductive dechlorination of PCBs (Field and Sierra-Alvarez, 2008).  For example, three phylotypes of Dehalococcoides spp. and a related Chloroflexi were detected in a microbial community that dehalogenated the predominant PCB congeners in Aroclor 1260 to tetra- and trichlorobiphenyls (Fagervold et al., 2007).  More recently, Adrian et al. (2009) demonstrated that Dehalococcoides sp. strain CBDB1 extensively dechlorinates Aroclor 1260.  While dechlorination patterns differ between strains, CENSUS quantification of Dehalococcoides spp. can be used to evaluate the potential for reductive dechlorination of PCBs.

 

Target

MI Code 

Relevance / Data Interpretation

Dehalococcoides  DHC While the range of compounds utilized varies, some Dehalococcoidesstrains are capable of reductive dechlorination of PCBs.
Dehalobium chlorocoercia DF-1 DECO Shown to reductively dechlorinate doubly flanked chlorines in PCBs.  Interestingly, in a mesocosm study, bioaugmentation with strain DF-1 appeared to also stimulate dechlorination of singly flanked chlorines by a seemingly synergistic effect on the indigenous microbial community (Payne et al. 2011).

 

Aerobic Degradation of PCBs

There are many examples of aerobic bacteria capable of cometabolic degradation of PCBs during growth on biphenyl.  Most isolates have been capable of degrading PCBs with only one or two chlorines, but there are exceptions.  Burkholderia xenovorans LB400, for example, is capable of cometabolism of some PCB congeners containing three, four, and five chlorines (Gibson et al., 1993).  Microbial Insights (MI) offers a set of CENSUS assays to quantify the genes encoding biphenyl dioxygenases (BPH) which are responsible for the first step in metabolism of biphenyl and cometabolism of lightly chlorinated PCBs.

 

 

Target

MI Code 

  Relevance / Data Interpretation

Biphenyl dioxygenase BPH Gene encoding the enzyme responsible for initiating aerobic cometabolism of PCBs.  The specificity of the biphenyl dioxygenase determines which PCBs are degraded.